The present invention relates to acoustic attenuation panels and more particularly those lining the annular fan channels formed in the nacelles of aircraft turbo engines in particular.
Such panels form the envelope, on the side of the fan channel, of the nacelle, behind the air inlet cowling and have a structure suitable to attenuate the noise produced by the motor surrounded by the nacelle and particularly the noise produced by the fan.
In practice, these panels integrate an open work core, such as a honeycomb sandwich structure, on the side of the fan channel, an acoustically resistive layer and, on the opposite side, a rear reflector.
The acoustically resistive layer plays a dissipating role, by partially transforming the acoustic energy of the sound wave passing through it, into heat.
This porous structure can be, for example, a woven cloth or a cloth of carbon fibers whose weave permits fulfilling its dissipating function.
As these acoustic panels must also have sufficient structural properties particularly to receive and transfer the aerodynamic and inertial forces and those connected with the securement of the nacelle, to the nacelle/motor structural connections, it is necessary to impart to the acoustically resistive layer structural properties.
To this end, one can, as shown by the patent GB 2 130 963, provide an acoustically resistive layer of two components, namely a structural layer, on the honeycomb side and a porous surface layer, or else use as a resistive layer a cloth combining both acoustic function and the structural function by selecting a diameter of the filaments of the cloth giving to this latter a high resistance to force coupled with good acoustical resistance.
For the production of such panels, the process is known consisting in providing the annular assembly forming the wall of the fan channel, in two half panels and two splints comprising, for each half panel, the following steps:
predeformation of a layer of porous structure on a form identical to a half panel with the help of jaws stretching the porous material to its elastic limit,
acoustic measurement of the shape thus produced so as to qualify the mean value of acoustic porosity,
fitting to the mean value above, of the winding interval of the carbon filaments that are to be deposited on the porous layer to constitute a structural layer,
emplacement on a suitable mold of the predeformed shape,
then producing the half panel by known winding techniques of said carbon filaments, and emplacing the porous core and the rear reflector.
This process has drawbacks.
Thus, the shape to be produced not being one of revolution, there exists in the deformed layer inhomogeneous zones, which is to say stretched zones and compressed zones, which degrade the overall acoustic quality of the porous structure. The winding interval of the carbon filaments being suitable for the means value of acoustic porosity of the structure, the inhomogeneous zones introduce variations in the acoustic attenuation of the noise generated by the motor.
Moreover, the presence of splints connecting the two half panels introduces discontinuities of impedance in the final acoustic panel, which is prejudicial to the quality of attenuation of the noises generated by the motor.
To overcome these drawbacks, one can, as taught by French patent 2 767 411 in the name of the applicant, emplace the acoustically resistive layer by wrapping with a porous material in the form of a strip.
This manner of operation not only permits avoiding inhomogeneous zones as to the acoustics of the porous layer in question, as indicated above, in the usual technique of producing two half panels, but also eliminates the need for splinting, the winding of the porous layer being able to take the place of the other layers, namely the structural layer, the central porous core, the rear reflector, so as to produce a complete acoustic panel in a single piece without splints.
The absence of splints permits increasing the effective acoustic surface of the panel, decreasing its mass and reducing the time and cost of fabrication.
If such a panel produces high quality acoustic damping, this damping is however not optimum. Thus, the physical characteristics of the resistive layer are homogeneous over all the extent of the panel and determined in consideration of the mean value of the noise which it is desired to damp.
However, the different modes of propagation of sound waves change in the course of their propagation in the fan channel. Thus, certain modes undergo high attenuation whilst others, on the contrary, are practically unaffected by the acoustic panel. Furthermore, the characteristics of the noise to be attenuated are different from one point to another in the channel. As a result, an acoustic attenuation panel of the type described above, attenuating only a limited number of modes, does not permit optimum damping of the noise.
Furthermore, the acoustic panels having for their object to attenuate the noise generated by the motor, it is preferred to attenuate the portion of the noise which radiates toward the ground. This requirement gives rise to a dissymmetric treatment of the problem of attenuation because the downward attenuation is favored relative to the upward.
The known panels are in no way adapted for such a differential acoustic treatment.
The present invention has precisely for its object to provide a process for production of acoustic attenuation panels of the above type, permitting adapting the acoustically resistive layer to the sonic environment of the panels so as to obtain a truly optimum damping.
To this end, the invention has for its object a process for the production of a panel with a shaped acoustically resistive layer, of the type comprising at least one central core with a faced porous structure, on the one side, a structural layer itself covered with a porous acoustically resistive layer and, on the other side, a total acoustic reflector, in which, successively:
there is wound stripwise said acoustically resistive layer on a mold shaped to the profile of the panel to be obtained,
said structural layer is emplaced,
said core with a porous structure is emplaced,
said total reflector is emplaced, then
the mold is withdrawn from the panel,
characterized in that in the course of depositing on the mold the acoustically resistive layer, the porosity of this latter is locally adapted to correspond with the characteristics of the sonic wave at the point of impact, the correlation between characteristic and impact zone being preliminarily determined with the help of conventional acoustic techniques applied to a test panel identical to that to be produced.
According to a first embodiment of practicing the procedure, there is first produced a continuous strip of acoustically resistive material constituted by end to end sections of different porosities and this strip is laid down on the mold, the porosity and length of each section as well as the manner of deposition being determined such that once the acoustically resistive layer is in place, the latter will be divided into at least as many contiguous regions as different porosities in each strip, in correspondence with the different predetermined acoustical regions in the course of said preliminary correlation carried out on the test panel.
According to a second embodiment of practice of the process, there is first provided a plurality of strips of acoustically resistant material of different porosities and these are laid on the mold successively in order, and the order of deposition is predetermined such that once the acoustically resistive layer is in place, the latter will be divided into at least as any contiguous zones as there are different porosities, in correspondence with the different acoustic zones determined in the course of said preliminary correlation carried out on the test panel.
According to a third manner of practicing the process, there is first provided a strip of an acoustically resistive material of the same nature but whose porosity varies longitudinally with the strip and this is laid on said mold, the ways of varying the porosity and of laying the strip being predetermined such that once the acoustically resistive layer is in place, the latter has local variations of porosity in correspondence with the different acoustic regions determined in the course of said preliminary correlation carried out with the test panel.
The invention also has for its object the panels obtained according to the above process, particularly acoustic attenuation panels for a fan channel, of a single piece without splints.